Contact binaries with additional components. I. The extant data

We have attempted to establish observational evidence for the presence of distant companions that may have acquired and/or absorbed angular momentum during the evolution of multiple systems, thus facilitating or enabling the formation of contact binaries. In this preliminary investigation we use several techniques ( some of them distance-independent) and mostly disregard the detection biases of individual techniques in an attempt to establish a lower limit to the frequency of triple systems. While the whole sample of 151 contact binary stars brighter than V-max = 10 mag gives a firm lower limit of 42%+/- 5%, the corresponding number for the much better observed northern-sky subsample is 59%+/- 8%. These estimates indicate that most contact binary stars exist in multiple systems.